1. Field of the Invention
The present invention relates to a thermal transfer sheet and in particular to a thermal transfer sheet excellent in retentivity of a heat fusible ink layer and transfer properties.
2. Description of the Related Art
In recent years, thermal transfer sheets utilizing a heat fusion transfer system are used for output print etc. of a computer, word processor etc. Generally, thermal transfer sheets utilizing a heat fusion transfer system are those in which a polyethylene terephthalate film of 3 to 20 .mu.m in thickness is used as the substrate and a heat fusible ink having a coloring agent dispersed in a binder is applied onto the substrate to form a heat fusible ink layer.
Conventional thermal transfer sheets include those using resin or wax as the binder of a heat fusible ink layer. The thermal transfer sheet provided with a heat fusible ink layer using resin as the binder is excellent in the adhesion properties (retentivity) of the heat fusible ink layer on the substrate, and the printed images formed have high friction resistance, but there are disadvantages of a poor transfer sensitivity, an inadequate compatibility with rough paper and a poor solvent resistance of printed images. On the other hand, the thermal transfer sheet provided with a heat fusible ink layer using wax as the binder is excellent in transfer sensitivity, can form good printed images on a rough paper and is excellent in the solvent resistance of printed images, but there are disadvantages of a poor retentivity of the heat fusible ink layer on the substrate and a low friction resistance of printed images.
Among the problems described above, the problem of retentivity can be solved by forming a primer layer between the substrate and the heat fusible ink layer to improve the adhesion properties therebetween, but in any cases where resin or wax is used as the binder, there are problems inherent in the material.
Accordingly, a thermal transfer sheet using both resin and wax as the binder to compensate for the disadvantages of both of them has been developed. For example, a thermal transfer sheet provided with a heat fusible ink layer using a petroleum resin as the resin and carnauba wax as the wax is used.
Even if both resin and wax are simultaneously used as the binder, the retentivity of the heat fusible ink layer on the substrate disagrees with the transfer sensitivity of the heat fusible ink layer, and therefore it is conceivable that the transfer sensitivity of the heat fusible ink layer is set at adequate levels and then a primer layer is formed between the substrate and the heat fusible ink layer in order to improve retentivity. However, the primer layer used heretofore suffers from the problem that the retentivity of the heat fusible ink layer using resin and wax as the binder cannot be raised to adequate levels for the substrate.
Further, it should be taken into consideration that sound upon detachment at the time of printing does not becomes too high in improving retentivity.